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The patterns


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Pattern: Event-driven architecture

Problem

You have applied the Database per Service pattern. Each service has its own database. Some business transactions, however, span multiple service so you need a mechanism to ensure data consistency across services.

For example, lets imagine that you are building an e-commerce store where customers have a credit limit. The application must ensure that a new order will not exceed the customer’s credit limit. Since Orders and Customers are in different databases the application cannot simply use a local ACID transaction. In theory, it could use a distributed transaction that spans the Customer Service and the Order Service. However, for a variety of reasons distributed transactions are not a viable choice for most modern applications.

Solution

Use an event-driven, eventually consistent approach. Each service publishes an event whenever it update it’s data. Other service subscribe to events. When an event is received, a service updates it’s data.

Resulting context

This pattern has the following benefits:

  • It enables an application to maintain data consistency across multiple services without using distributed transactions

This solution has the following drawbacks:

  • The programming model is more complex

There are also the following issues to address:

  • In order to be reliable, an application must atomically update its database and publish an event. It cannot use the traditional mechanism of a distributed transaction that spans the database and the message broker. Instead, it must use one the patterns listed below.

Example

An e-commerce application that uses this approach would work as follows:

  1. The Order Service creates an Order in a pending state and publishes an OrderCreated event.
  2. The Customer Service receives the event and attempts to reserve credit for that Order. It then publishes either a Credit Reserved event or a CreditLimitExceeded event.
  3. The Order Service receives the event from the Customer Service and changes the state of the order to either approved or cancelled

See also

The article Event-Driven Data Management for Microservices by @crichardson describes this pattern


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